If you assume that marketing is about dialog between entities and individuals, then cause marketing is no different from any other kind of marketing.

The beauty of the Internet is that it handles long tail situations extremely well. Everyone is as close as their mouse. So provided you can get visible on the Internet (marketing), then you may be able to establish contact with 50, 200, 2000 people around the world who share your interests.

Is a green business's green focus just another Unique Selling Position?

While I doubt the definitive study on this has been done, I would place it in a special category. Green is much more holistic, comprehensive and prone to backfire.

Hardly anyone is a better poster boy than Al Gore. As you may remember, Gore came under criticism for things like a less than enviro-friendly house.

In other words, "green" is more like brand positioning than a USP. The entire point of a USP is to do something others don't -- not jump on the bandwagon. And especially don't jump on the bandwagon with only one foot. Many companies open themselves up to charges of superficial green gimmickry.

A branding campaign is more in line with how this has to happen. For example, giving a percentage of sales to green causes, coupled with product and packaging redesign, and potentially right on down to the decision of corporate headquarters.

In other words, if you have an office in an energy wasting building, drive an SUV, and drink bottled water, you're 1) Hardly unique -- the U in USP is out the window 2) A balloon in search of a pin, vulnerable to being called a hypocrite or worse

So, what can you do? Well, you can do any number of things where you, for example do those seed packet business cards. None of that outright claims to be a "green USP" but gives recipients a nice "afterglow."

The only way to make this into a USP would be to pioneer a business model so extreme green it frightens your competitors.

You've opened yourself up to questions about how green you are, and are you green enough.

William McDonough and Cradle-to-Cradle design. You can't even begin to imagine how throughly ungreen every single element of the economy is, even now. With his Cradle-to-Cradle design philosophy for the next industrial revolution McDounough has one of the only sane philosophies out there on design.

In other words, green isn't suited to the cheap sloganeering and Non-USP USPs people gravitate to.

I can't say much about "green" because I spent most of my lifetime in one of the "brown" professions... however, (and drawing a little upon that experience) I am willing to bet that if you have an obvious "green" theme to your site that it will not hurt you with anyone. And, if someone with that inclination lands on your site - even by accident - they would have a higher probability of looking around to see what you have.